Terahertz waves are electromagnetic waves with a frequency of approximately 0.01 THz to 1000 THz corresponding to an intermediate region between light waves and radio waves, and have intermediate characteristics between light waves and radio waves. As application of such a terahertz wave, a technique for acquiring information on an object to be measured by measuring a time waveform of an electric field amplitude of a terahertz wave transmitted through or reflected by the object to be measured (refer to Non-patent documents 1 to 4) has been studied.
The measuring technique for information on an object to be measured by using a terahertz wave is generally as follows. That is, pulsed light output from a light source (for example, a femtosecond laser light source) is two-separated into a pump light and a probe light by a separator. The pump light of these is input into a terahertz wave generating nonlinear optical crystal, and accordingly, a pulse terahertz wave is generated from this nonlinear optical crystal. The generated terahertz wave is transmitted through or reflected by an object to be measured, and accordingly, information on the object to be measured (for example, an absorption coefficient, and a refractive index) are obtained, and thereafter, the wave is synthesized with the probe light by a wave synthesizer, and made incident on a terahertz wave detecting electro-optic crystal at a timing substantially synchronous with the probe light.
In the electro-optic crystal into which the terahertz wave and the probe light are input, birefringence is induced along with propagation of the terahertz wave, and the birefringence changes a polarized state of the probe light in the electro-optic crystal. Therefore, a polarizer is provided on the light path of the probe light between the separator and wave synthesizer, an analyzer is provided on the output side of the electro-optic crystal, and the intensity of probe light transmitted through the analyzer is detected, whereby polarized state changes of the probe light in the electro-optic crystal are detected, and eventually, an electric field amplitude of the terahertz wave is detected, and characteristics of the object to be measured are obtained. Thus, the measuring technique for information on an object to be measured by using a terahertz wave uses an electro-optic effect caused by a pulse terahertz wave on the terahertz wave detecting electro-optic crystal.
Generally, the pulse width of a terahertz wave is on the level of picoseconds, and on the other hand, the pulse width of probe light is on the level of femtoseconds, so that the pulse width of probe light is several digits smaller than that of a terahertz wave. Accordingly, by sweeping an incidence timing of the probe light on a terahertz wave detecting electro-optic crystal, a time waveform of an electric field amplitude of a pulse terahertz wave is obtained.    Non-patent document 1: Jie Shan, et al., Opt. Lett., Vol. 25, No. 6, pp. 426-428 (2000)    Non-patent document 2: Takashi Yasuda, et al., Optical Terahertz Science and Technology (OSA Topical Meeting), Orlando, USA, March 14-16, Technical Digest (CD) TuC6 (2005)    Non-patent document 3: Zhiping Jiang, et al., Appl. Phys. Lett. Vol. 72, No. 16, pp. 1945-1947 (1998)    Non-patent document 4: Zhiping Jiang, et al., Opt. Lett., Vol. 24, No. 16, pp. 1245-1947 (1999)